Load Distribution Crown

ABSTRACT

An apparatus for distributing the load caused by the insertion of a fastening device into bony tissue is disclosed. In one embodiment, the apparatus comprises a crown having a plurality of protrusions. The protrusions may have any desired physical properties, and function to obtain purchase in the bony tissue. Preferably, the crown includes an opening that is selectively engageable with a spherical head of the fastening device. The opening preferably comprises a spherical curvature that allows the surfaces of the spherical head and the opening to maintain flush contact, even when the fastening device is inserted into the bony tissue at a non-orthogonal angle. When the fastening device is inserted into the bony tissue, the crown distributes the load over a wider section of the bone, allowing the bone to maintain its structural integrity.

FIELD OF THE INVENTION

The present invention relates to fastening devices used with bonytissue. More specifically, the present invention relates to an apparatusthat distributes the load of a fastening device in order to prevent thestructural integrity of the bony tissue from being compromised.

BACKGROUND OF THE INVENTION

The field of medical engineering is rapidly expanding due to advances inengineering, material science, and advances in the field of medicine.These advances have allowed doctors and engineers to work together tocreate instruments and devices that allow more accurate and effectivediagnosis and treatment of a wide variety of ailments. The diagnoses andtreatments enable patients to recover from injuries or diseases thatused to be crippling, or even fatal.

One area of the human body that is of particular importance is theskeletal system. Advances in medical engineering and medicine haveallowed doctors to treat a wide variety of ailments involving theskeletal system, including back, neck, and joint pain. In addition,advancements have allowed faster healing of breaks and fractures ofbones. Of particular importance are breaks or fractures of the spinalcord, because they can result in severe pain or even paralysis.

One common method of fixing broken or fractured bones is through the useof a fixation device. Typically, this involves a screw or a nail beinginserted into the bone. Often, the screw or nail functions to anchorother components of a medical device to the bone. Additionally, afastener and fixation device may be used to join two parts of a bone, orto hold two bones together. For example, bone screws may be used in aspinal fixation system to immobilize two or more vertebrae. Thesefixation devices can be used in conjunction with other devices orcomponents, such as plates that hold the bones together and preventmovement.

Despite the advances in medical engineering, fixation and other devicesare susceptible to the inherent weaknesses of bones and other bonytissue. Further complicating the matter, bones may become weak, brittle,or lose strength for a number of reasons. For example, the patient's ageand nutrition may be factors for bone strength. Additionally, thestrength of the bones may vary based on the degree of damage to the bonefrom disease, trauma, or the like. As the structure or strength of abone lessens, it becomes increasingly difficult to use it for placementof a fastener. Thus, a bony anatomy capable of receiving and securelyholding a fastener when healthy may not be sufficiently strong enough toresist pressure from a screw head when in a weakened state. Because afixation device requires penetration of the bone, the strength of thebone has to be considered. The insertion of fixation devices, such asscrews, often requires the drilling of holes into the bone. As the headof the fastener contacts the bony anatomy, it can impart a localizedconcentration of stress or pressure that a weakened bone may not be ableto withstand. The exertion of a significant load on a small area of thebone may cause it to chip, crack, fracture, collapse, or break.

When fixation devices are used in spinal applications, the movement ofthe vertebrae must also be taken into consideration. A significantproblem with screws that are inserted into vertebrae is that they tendto “backout,” or unscrew due to the motion of the bones. When a screwbacks out, it can require additional invasive procedures in order tocorrect the problem. In addition, because vertebrae vary in shape andsize, they are particularly susceptible to breaking or cracking.

A continuing need exists for a load distribution device that is able toaccommodate screws at various angles of insertion. Furthermore, a needexists for a load distribution device that is able to prevent backingout of a fixation device and reduce the likelihood of undesiredchipping, cracking, fracturing, or breaking of the bone when thefixation device is being inserted.

SUMMARY OF THE INVENTION

The present invention relates to a crown that distributes the load of afastening device in order to reduce the chances of the structuralintegrity of the bone being undesirably compromised. The presentinvention may be used with any type of fastening devices, such asscrews, nails, or the like that have any desired physicalcharacteristics. The fastening device is preferably inserted through anopening in the load distribution crown. Preferably, both, the fasteningdevice and the crown are configured and dimensioned to havecomplimentary interfaces. The complimentary interfaces allow the twosurfaces to rest flush against one another. One advantage of allowingthe two surfaces to be flush is that the load may be more evenlydistributed over the bottom surface, or base, of the crown facing thebone in which the fastener is inserted. Another advantage of thecomplimentary interfaces is that the load may be distributed evenly evenwhen the screw or fastener is inserted at a non-orthogonal angle.

In one embodiment, the bottom face of the crown lies substantially flushwith the surface of the bone. In another embodiment, the crown isconfigured to have a slit or cut extending from its outer perimeter edgeto the inner perimeter edge that defines the opening through which thefastener is inserted. The cut or opening may be further defined by twofree ends of the crown. When a slit or cut is present, one end of thecrown that defines the cut or slit may be disposed higher than theopposing end of the crown. As the fastener is inserted through theopening and into the bone, the head of the fastener may apply pressureto the surface of the crown and cause the crown to, flex or bend,thereby causing the free ends of the crown to move toward a commonplane. In one embodiment, the free ends of the crown may move from about0.1 mm to about 5 mm before reaching a common plane. In anotherembodiment, the free ends may move from about 0.5 mm to about 3 mmbefore reaching a common plane. The flexing of the material that formsthe crown allows for the storage of potential energy in the crown thatcan be released if the screw backs out. In addition, the use of a slitor cut allows for gradual increasing and decreasing of load distributionby the crown as the fastener is inserted into or removed from the bone.

In one embodiment where a cut or slit is provided in the loaddistribution crown, the free ends of the crown may be configured toprovide a tactile or audible signal to the physician about the degree towhich the crown has flexed or deformed. Thus, in one embodiment of theinvention, the free ends of the crown are configured to provide three ormore audible signals to the physician. In one embodiment, at least onefree end of the crown has at least one detent or protrusion thatinterferes with a portion of the opposing free end of the crown. As thefree ends move toward a common plane, the detent or protrusion iscapable of providing tactile or audible signal to the physician.

In one embodiment, the opposing free end of the crown has one or moredetents configured such that moving the detent from the first free endof the crown past one or more detents of the opposing free end causes atactile, or audible signal. In yet another embodiment, the opposing endmay have one or more recesses or channels such that moving the detentfrom the first free end of the crown past one or more recesses orchannels of the opposing end causes a tactile or audible signal. Someembodiments of the invention may combine one or more detents andrecesses or channels on the same free end to provide a variety ofsignals. In one embodiment, the potential energy stored in the crownfrom its deflection is sufficient to overcome the interference caused byinterferences between the free ends that provide signals to thephysician. Thus, in some embodiments the crown is capable of at leastpartially moving back toward its initial configuration if a fastener isbacked out or if pressure on the crown is relieved for any reason.

In yet another embodiment where the crown is configured to provide aplurality of tactile or audible signals, the pressure or force needed tocreate one signal is greater than the pressure or force needed to createanother signal. For example, in one embodiment, the force or pressureneeded to be applied by the fastener to the crown to move the free endsof the crown to create an initial signal is less than the force orpressure needed to be applied by the fastener to the crown to move thefree ends of the crown to create a second signal.

In yet another embodiment where the crown is configured to provide aplurality of tactile or audible signals, the intensity of one signal isgreater than the intensity of another signal. For example, if the signalprovided is an audible “click” the signals may progressively increase inintensity to notify the physician of the degree of compression of thecrown between the fastener head and the bone. Thus, in one embodiment, afinal signal during compression may have the greatest intensity so thata physician is notified that further deployment of the fastener may riskdamage to the bone or loss of purchase of the fastener in the bone.

To further promote bone contact over a substantially portion of thebottom surface of the crown, the crown may have protrusions that allowit to achieve contact or purchase with the bone even when the bonysurface contacting the crown is uneven or has a depression in it, oreven when the crown with a cut or slit has not deflected sufficientlyfor the bottom surface to fully contact the bone. One or moreprotrusions disposed over a location of the bony anatomy that is sunkenor lower than other portions of the bone may be used to contact theselower portions of the surface.

One advantage of using protrusions is that the screw or fastener may bemore likely to stay in place when used in combination with a crown. Inaddition, the protrusions may help minimize shearing or grinding of thebone during insertion of the screw. In one embodiment of the invention,a portion of the crown and screw have complimentary interfacing surfacesthat permit the fastener to rotate or swivel relative to the crown. Thisallows the bottom surface of the crown to achieve contact with the boneeven if the fastener is inserted of angle from an orthogonal direction.

Additionally, the fastener and crown may be configured so that portionsof the complimentary surfaces initially interfere with each other untila threshold force between them is achieved. As the screw or fastener isurged to move further into the bone, the interference area may cause thecrown to be pressed into the bone. Eventually, these downward forcesexceed the force needed to push the fastener head beyond theinterference region and into a receptacle of the crown having acomplimentary curved surface corresponding to the surface of thefastener head.

In one embodiment, the present invention comprises an apparatus fordistributing a fastener head load over an increased surface of the bone.The apparatus comprises a crown comprising one or more protrusions andan opening having a generally spherical curvature. The crown maycomprise any desired material. Also included is a fastener including ahead that may initially interfere with the spherical curvature of theopening as described above. Alternatively, the opening of the crown thatreceives the fastener head may be wide enough for the complimentarysliding surfaces to contact each other without passing through aninterference area. The fastener is selectively engageable with theopening to allow the crown to distribute the load caused by theinsertion of the fastener into bony tissue. In one embodiment, theradial loading caused by the torque required to insert the fastener isborne by the crown. The crown then transfers the radial loading to axialloading that is applied to the bone.

As noted above, the bottom surface of the crown that contacts bone maybe configured with one or more protrusions, or alternatively may besubstantially free of protrusions. When protrusions are used, it may bedesirable to alter the length, concentration, and distribution of theprotrusions depending upon the type of bone that the crown will contact.In one embodiment, the length of the protrusions is preferably betweenabout 0.7 and about 1.2 mm. The concentration of the protrusions, whichcan be measured by the average peak-to-peak distance between the one ormore protrusions, is between about 0.7 mm to about 1.0 mm. Another wayto measure the concentration of the protrusions is by the average baseto base distance between neighboring protrusions, which may be betweenabout 0.5 and 1.0 mm. Thus, the protrusions may be sufficientlyseparated to provide flat surfaces on the bottom surface. Alternatively,the protrusions may form a sawtooth pattern.

Additionally, the shape of the protrusions may be configured and adaptedfor a particular application. In one embodiment, the protrusionscomprise an angle. In such an embodiment, the angle from a base of theprotrusion to a peak of the protrusion may be between about 55 degreesand about 65 degrees.

In some embodiments, the spherical curvature of the complimentarysurface of the crown is configured and dimensioned to evenly distributethe load caused by the insertion of the fastener into the bony tissue ata non-orthogonal angle. In one embodiment, the tangents to the sphericalcurvature of the opening intersect. Depending on the size of the curvedsurface of the fastener head, this crown configuration may result ininterference between the spherical head and the spherical curvature ofthe opening.

In another embodiment, the complementary surfaces of the crown andfastener head may be configured with a plurality of ridges, steps, orledges extending substantially in a direction orthogonal to therotational direction of the fastener. As the ridged surfaces of thefastener and crown begin to engage, and ultimately reach their finalposition, the ridges may help resist forces from micromotion orvibration that might cause the fastener to back out of its position.

In order to effectively distribute the load caused by the fastener, thetotal surface area of the base of the crown may be about 10 square mm orgreater. In some embodiments, it may be desirable to deform the surfacearea of the base in order to match the contour of the base to thecontour of the bone involved. In such embodiments, the force necessaryto deform the crown is preferably substantially greater than the forcenecessary to insert the screw into the bony tissue. For instance, theforce needed to bend or deform the crown may be about 5 times or greaterthan forces that may be imparted on the crown by the fastener duringinsertion into bone. This prevents the crown from being deformed duringthe insertion of the screw. In some embodiments, an incision, cut, orslot may be formed in either the top or base to aid in deforming thecrown at one or more predetermined locations.

It is desirable to prevent the fastener from passing completely throughthe crown. This may be achieved by making the diameter of the crownopening, through which a threaded portion of a screw or body portion ofa fastener may pass, at least between about 5% and 10% smaller than thediameter of the head of the fastener. To permit rotation or swiveling ofthe fastener relative to the crown, the diameter of the fastener may befrom about 5% to about 30% less than the diameter of the diameter of thecrown opening.

In another embodiment, the present invention comprises an apparatus fordistributing a load in bony tissue. The apparatus comprises a fastenerand a crown comprising an opening. The fastener is selectivelyengageable with the opening to allow the crown to distribute the loadcaused by the insertion of the fastener into bony tissue. In thisembodiment, the crown may include at least one protrusion. In otherembodiments, the number of protrusions may be about 30 or less.Furthermore, the length of the protrusions may be from about 0.1 mm toabout 4 mm from tip or peak to base. The concentration of theprotrusions may be measured by the average peak-to-peak distance betweenthe one or more protrusions, which may be between about 0.4 mm to about2.5 mm. Alternatively, the average base to base distance betweenneighboring protrusions may be between about 0.1 mm and about 1.5 mm.Some of the protrusions may comprise an angle wherein the angle from abase of the protrusion to a peak of the protrusion is between about 50degrees and about 70 degrees.

In another embodiment, the present invention comprises an apparatus fordistributing a load in a spinal fixation system. The apparatus comprisesa fastener and a crown including a plurality of protrusions and anopening. The opening is configured and dimensioned to distribute theload caused by insertion of the fastener into bony tissue.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a side view of the crown according to anexemplary embodiment of the present invention;

FIGS. 2A-2C are diagrams showing exemplary protrusions according to thepresent invention;

FIGS. 3A-3C are diagrams showing exemplary distributions of theprotrusions;

FIGS. 4A-4E are diagrams showing exemplary geometric configurations ofthe crown;

FIGS. 5A-5D are diagrams showing incisions according to an exemplaryembodiment of the present invention;

FIG. 6 is a diagram showing a screw according to an exemplary embodimentof the present invention;

FIGS. 7A-7C are diagrams illustrating the spherical curvature of theopening of the crown; and

FIGS. 8A-8D are illustrations of embodiments of the present inventionhaving a spring mechanism capable of deflecting to provide graduallyincreasing or decreasing pressure on the bone in response to fastenermovement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fastening devices are currently used in a wide variety of applicationsthat range from construction to medical equipment. Typical fasteningdevices include nails and screws. Screws are often used as fasteningdevices in medical applications, typically to fasten a device to bonytissue or to provide temporary or permanent support.

In medical applications, one consideration when using a screw is theamount of force that the screw will exert on the bony tissue that it isinserted into. If a screw exerts too much pressure on bony tissue, itcan compromise the structural integrity of the bone, which can causemany complications. The angle of insertion can also compromise thestructural integrity of the bone. In some instances if the angle ofinsertion diverges significantly from a direction orthogonal to the boneit may cause the bone to crack or break. Thus, the angle of insertionand the pressure that a screw exerts are factors to consider when usedin medical applications.

The present invention relates to a crown that that distributes the loadof a fastening device in order to reduce the chances of the structuralintegrity of the bone being compromised. The present invention may beused with any type of fastening devices, such as screws, that have anydesired physical characteristics. The fastening device is preferablyinserted through the crown. Preferably, both the fastening device andthe crown are configured and dimensioned to have complimentaryinterfaces. For instance, the complimentary surfaces may both havespherical curvatures and the radius of curvature for each surface may besubstantially the same as the other such that contact between thesesurfaces is over an area instead of a point or line. Thus, it ispreferred that the complimentary interfaces allow the two surfaces tomaintain flush contact with one another yet also permit a range ormotion or angulation between them to account for variations in the angleat which a fastener is inserted into the bone. One advantage of allowingthe two surfaces to be flush is that the load may be evenly distributedover the bottom surface of the crown. Another advantage of thecomplimentary interfaces is that the load may be distributed evenly evenwhen the screw is inserted at an angle.

It is desired that the base of the crown lies flush with the surface ofthe bone. The crown may have protrusions that allow the crown to achievepurchase in the bone. One advantage of the protrusions is that the screwis more likely to stay in place when used in combination with the crownthan only a threaded screw. Another advantage of the protrusions is thatthey minimize grinding of the bone during insertion of the screw. Tofurther decrease the chance of the screw backing out, the portion of thecrown and screw that interface with each other may include complimentaryuneven surfaces. The complimentary uneven surfaces create interference,which has to be overcome in order for the screw to be removed.

In one embodiment, the present invention may be used with any type ofbony tissue, in any part of the body. For example, the present inventionmay be used in the spine, in long bones, in the skull, or in other partsof the skeletal system. In some embodiments, the shape of the surface ofthe crown that contacts the bone may be configured and dimensioned torest flush with the surface of the bone. Accordingly, the crown may beused with a bone having any shape, for example, flat, round, irregular,or the like. In a preferred embodiment, the present invention is usedfor applications involving the spine. In such embodiments, the crown andfastener may be used with other devices to form, for example, a spinalfixation system.

FIG. 1 is a diagram showing a side view of a crown 20 according to anexemplary embodiment of the present invention. As shown in FIG. 1, thecrown may have a plurality of protrusions 22 selectively positioned atthe base of the crown. The protrusions 22 are preferably located on thebase of the crown, which rests against the surface of the bone. In suchan embodiment, the protrusions 22 allow the crown to achieve purchase inthe bone as well as potentially accounting for variations or depressionsin the surface of the bone. The protrusions may be varied in any aspect,such as size, shape, distribution, and the like, as described in moredetail below.

In one embodiment, the protrusions 22 may be configured and dimensionedin any desired manner. In one embodiment, for example, the length of theprotrusions may be varied. It may be desirable to vary the length of theprotrusions 22 in order to achieve a desired amount of purchase in thebone. Alternately, it may be preferable to vary the length of theprotrusions 22 based on the size, shape, or type of bone that isinvolved. In other embodiments, it may be desirable to vary the lengthof the protrusions 22 based on the region of the spine that is involved.For example, protrusions 22 for treating the cervical region may beshorter in length than protrusions 22 for treating the lumbar region.

The length or height of the protrusions 22 may be described according tothe distance from the base of the protrusion to the tip, peak, orportion of the protrusion 22 that is furthest from the base. Preferably,the length of the protrusions is between about 0.1 and about 4 mm. Morepreferably, the length of the protrusions is between about 0.5 and about2 mm, and most preferably the length of the protrusions is between about0.7 and about 1.2 mm.

In one embodiment, a desired amount of purchase may be achieved byvarying the number of protrusions 22 that are present. Generally, moreprotrusions 22 may yield better gripping of the bone under certainconditions, such as when the bony surface is relatively flat. However,if there are too many protrusions 22, the structural integrity of thebone may be compromised when they are driven into the bone. Plus, theforce needed to drive the protrusions 22 into the bone may increasesignificantly. Furthermore, an abundance of protrusions may make thecrown less capable of engaging with or contacting bony regions that aredepressed or lower than other portions of the surface. Thus, it isdesirable to choose the number, concentration, and distribution ofprotrusions 22 such that a sufficient purchase in the bone may beachieved without compromising the structural integrity of the bone.Preferably, the number of protrusions is about 40 or less. Morepreferably, the number of protrusions is about 20 or less, and mostpreferably the number of protrusions is about 10 or less.

Another way to achieve a desired amount of purchase in the bone is tovary the geometric shape of the protrusions 22. FIGS. 2A-2C are diagramsshowing exemplary protrusions according to the present invention. Insome embodiments, it may be desirable to use conically shapedprotrusions, as shown in FIG. 2A. Alternately, pyramid shapedprotrusions, shown in FIG. 2B may be desired. The shape of theprotrusions shown in FIGS. 2A-2B may be preferred when it is desirablefor the crown to avoid motion once the screw has been completely driveninto the bone or where the bony surface may be uneven. In other words,once the screw has been driven into the bone, the protrusions may atleast partially pierce and extend into the bony surface.

In another embodiment, it may be preferable for each protrusion 22 toachieve a maximum amount of purchase in the bone by providingprotrusions that more readily permits sliding in one direction whilemore substantially resisting motion in a second direction. Thus, angled(FIG. 2C) or curved protrusions may be used, and a plurality of them maybe oriented on the bottom surface of the crown to permit rotationalsliding in one direction more readily than in another direction. Oneadvantage of an angled or curved protrusion is that it can be driveninto the bone by applying a given amount of torque. Another advantage ofan angled or curved protrusion is that it may be configured anddimensioned such that it provides more resistance to motion in onedirection than in the other direction. Thus, when used in combinationwith complimentary interfering surfaces or textured surfaces, both ofwhich are discussed in more detail below, the angled or curvedprotrusions may help prevent a fastener or screw from inadvertentlycoming loose or backing out.

As mentioned above, in some embodiments protrusions may comprise one ormore angled edges. One way to describe these edges is by the angle fromthe point to the base of the protrusion 22. In some embodiments, theoverall configuration, distribution, and number of protrusions may varydepending on the intended location in the spine where the crown may beinserted. For instance, some or all of the protrusions may have asawtooth configuration having substantially symmetrical surfaces andangles off of a vertical axis that extends through the peak of theprotrusion. Alternatively, as described above, the protrusions may havedifferent angles off of the vertical axis so that the protrusions aremore resistant to movement in one direction than in the oppositedirection. In either case, the angle at which the side or sides of aprotrusion extend from the peak to a base may form an angle of about 50°to about 70°, and preferably is about 60°.

In addition to the length and geometric shape of the protrusions 22, thedistribution of the protrusions around the base of the crown may bevaried. FIGS. 3A-3C are views of the bottom surfaces of crowns, whichultimately will be pressed against the surface of the bone to distributeloading by the fastener over a greater area than the fastener head. Thedifferent views illustrate some of the different, non-limitingdistributions or protrusions that may be used.

In one embodiment, the distribution of the protrusions may be variedaccording to the radial distance from the center of the crown. Forexample, the distribution of the protrusions may be varied over tworegions 30-32, as shown in FIG. 3A. For example, protrusions in theouter circumferential region may be longer than the protrusions in theinner region. For instance, the average length of the protrusions in theouter region may be from about 5% to about 30% longer than the averagelength of protrusions in the inner region. In this manner, it isbelieved that the bony surface beneath this outer region will be morelikely to be partially supporting the loading of the fastener. It shouldbe understood that more than two regions of protrusions may be used aswell. Alternatively, the distribution may be varied on each half 34-36of the crown, as shown in FIG. 3B. In other embodiments, there may be adifferent distribution in each of several portions 38, 40, 42, and 44 ofthe crown, shown in FIG. 3C.

In some embodiments, such as the ones shown in FIGS. 3A-3C, the physicalcharacteristics of the protrusions, such as length, shape, and the like,may be different in each region. In some embodiments it may be desirableto have different sized or shaped protrusions in each region. Forinstance, it may be preferable to have the protrusions in outer region32 that are, for example, larger, longer, or sharper than theprotrusions located in inner region 30. The protrusions in outer region32 may be different than the ones located in the inner region 30 so toensure that region 32 achieves sufficient purchase in the bone.

In one embodiment, the distribution of the protrusions 22 may be variedbased on the type of bone that is involved. In these embodiments, it isdesirable to vary the concentration of the protrusions. In other words,the number of protrusions for a given area may be increased ordecreased, based on the type or shape of bone involved. For instance,when used in applications involving the lumbar region of the spine, agreater concentration of protrusions 22 may be desired. In contrast, acrown used in the cervical regions of the spine may have a lowerconcentration of protrusions. Not only may the protrusions 22 for thecervical region be less concentrated, but they also may be smaller inheight that the protrusions that might be used for the lumbar regions.

One benefit a lower concentration of protrusions 22 can provide fortreating a cervical region is that there is greater likelihood that theprotrusions will fully or at least substantially extend into and gripthe treated area in order to resist movement. In these embodiments, theportions of the crown where the protrusions 22 are located may not restagainst the bone. This can occur when, for example, the bone isirregularly shaped or if it is not substantially flat. In contrast, whenthe spacer is in the lumbar region it may be more beneficial to have ahigher concentration of protrusions in order to account for variationsin anatomy between different patients. In general, the anatomy ofvertebral bodies in the lumbar region may be curved, but the curvatureof a vertebral body from one patient to another may vary. By having theprotrusions densely packed together, any potential adverse effect onproperly positioning the crown that may result from these differences incurvature can be minimized.

One way to describe the concentration of protrusions 22 on the crown maybe by the average distance between the peak of a first protrusion to aneighboring one. For instance, a crown used in an application fortreating the lumbar region of the spine may have an average peak-to-peakdistance of about 0.4 mm to about 2.5 mm. In another embodiment, theaverage peak-to-peak value is from about 0.7 mm to about 1.0 mm.Similarly, one embodiment of a crown used in an application for treatingthe cervical region of the spine may have a peak-to-peak distance ofabout 0.3 mm to about 2.5 mm, while in another embodiment the distancemay be from about 1.0 mm to about 1.5 mm. Preferably, the peak-to-peakdistance is about 1.25 mm when the crown is intended to be used inapplications that treat the cervical region of the spine.

Another way to describe the concentration of protrusions 22 on the baseof the crown is by the average spacing between the bases of a firstprotrusion to a neighboring one. As protrusions 22 are spread fartherapart, the surface of the crown in between the protrusions becomeslarger. Thus, when a crown is used in applications for treating thecervical region of the spine, the average distance between the bases ofthe protrusions may be about from 0.1 mm to about 1.5 mm. In otherembodiment, the average base-to-base distance between protrusions isfrom about 0.5 mm to about 1.0 mm. Preferably, the average base-to-basedistance between neighboring protrusions is about 0.8 mm. As mentionedabove, the protrusions 22 on a crown used in applications that treat thelumbar region of the spine may be located more closely to each otherthan the protrusions on a crown for other regions of the spine. Forinstance, the average distance between the bases of the protrusions maybe from about 0 mm to about 1.0 mm. In another embodiment the averagebase-to-base distance is less than about 0.5 mm, and more preferably isless than about 0.25 mm.

In some embodiments, it may be desirable to determine the physicalcharacteristics of the protrusions 22 based on their desired function.For example, in some embodiments it may be desirable to have theprotrusions 22 partially pierce the bone to obtain a desired amount ofpurchase. The purchase that is achieved preferably prevents slippage anddistributes the load as the screw is driven into the bone. When anexcessive amount of force is exerted on the bone by the screw, thestructural integrity of the bone may be compromised. One advantage ofthe protrusions 22 achieving partial purchase is that the load can bedistributed prior to the screw being fully inserted into the bone. Thispreferably decreases the chance of the structural integrity of the bonebeing compromised by the force of inserting the screw. In addition, thepartial purchase achieved by the protrusions 22 can provide stabilityfor the screw. One advantage of the stability is that the crown is ableto provide a guide for inserting the screw. This may be particularlyimportant in applications where it is desirable to avoid interferencebetween two or more inserted screws. These are just examples, and arenot intended to limit the present invention. In other embodiments, thephysical characteristics of the protrusions 22 can be chosen to performa plurality of other functions, such as providing visual cues to asurgeon indicating that the screw is fully inserted.

Turning now to FIGS. 4A-4D, the physical characteristics of the crownare described. As mentioned above, one advantage of the presentinvention is that the load generated by the screw can be distributedover a larger surface area of bone such as over an area of at leastabout twice the area of the fastener screw head that would otherwisecontact the bone. This prevents the structural integrity of the bonefrom being compromised from cracking, breaking, chipping, and the like.It is highly desirable to avoid these occurrences because it can resultin a plurality of complications, such as longer healing time, longer ormultiple surgeries, failure of the fastener to be securely held inposition, and the like.

The load caused by the screw may be more widely distributed byincreasing the overall area of the base of the crown. By increasing thearea of the base, a larger portion of the crown may come into contactwith the bone, thereby distributing the load over a larger area thanwould be expected from the fastener or screw head alone. However,practical applications require the size of the crown to be limited inorder to, for example, fit within the body, be suitable for use withother devices, limit the profile of the combined screw and crown, and beused with bones of varying size and shape.

When considering the area over which the load of the screw may bedistributed, only the surface area of the base of the crown should beconsidered. In some embodiments, the surface area of the base of thecrown may be different than the surface area of the top of the crown.However, the load is only distributed over a surface area of the crownthat is in contact with the surface of the bone. One way to describe thesurface area of the base of the crown is by the total area that iscapable of contact with a surface, such as a bone. Preferably, the totalsurface area of the base of the crown is about 10 mm² or greater. Morepreferably, the total surface area of the base of the crown is about 20mm² or greater, and most preferably the total surface area of the baseof the crown is about 30 mm² or greater.

In one embodiment, the shape of the crown may comprise any desiredgeometry. In some embodiments, as mentioned above, the shape of the topposition of the crown and the shape of the base of the crown may bedifferent. In these embodiments, it may be desirable to determine theshape of the bottom portion of the crown based on the desired surfacearea that is in contact with the surface of the bone. The geometry ofthe top of the crown is discussed in greater detail below. However, insome embodiments, it may be desirable to have the geometry of the topand bottom surfaces substantially similar.

In one embodiment, the desired surface area of the base of the crown maybe achieved using a plurality of shapes. In a preferred embodiment, acircular crown including a circular opening may be used, as shown inFIG. 4A. One advantage of a circular crown including a circular openingis that they are easy to manufacture. In addition, a substantiallycircular crown is believed to provide greater surface area coverage in amore compact configuration than may be achieved from other shapes.

Another advantage of the circular crown is that the screw, whose shaftis typically substantially circular, can easily fit through the circularopening. In some embodiments, the inner surface 48 may comprise threadsthat correspond to threads located on the surface of the shaft of thescrew. In these embodiments, it is preferable to have a circular screwhaving a circular opening in order to correspond to the shape of theshaft of the screw.

However, the base of the crown is not limited to any particular shape.In some embodiments, it may be desirable to have a square shaped basewith a square opening, as shown in FIG. 4B. In other embodiments, it maybe desirable to have the base of the crown comprise a triangular shapehaving a triangular opening, as shown in FIG. 4C. In the FIGS. 4B and 4Cembodiments, the size of the opening is preferably configured andadapted such that it is able to pass the screw with a substantiallysmall amount of resistance. In another embodiment, the shape of the baseof the crown may be configured according to the expected neighboringsurface of the bone. For example, as shown in FIG. 4D, at least thebottom portion of the crown 50 may have a generally cylindricalcurvature when viewed from a side or otherwise may be configuredaccording to the shape of the bone 52. In such embodiments the geometryof the top portion of the crown does not have to be substantiallysimilar to the bottom portion of the crown. However, this may bedesirable in some applications. In other embodiments, the shape of theouter perimeter of the base of the crown may be substantially differentthan the shape of the opening, as shown in FIGS. 4C and 4E.

In some applications, it may be desirable to allow a surgeon to changethe geometry of some or all portions of the crown based on the shape ofthe bone. This provides the advantage of allowing a surgeon to visuallyinspect the bone and configure the shape of the crown accordingly. Asdiscussed above, this may be desirable in order to further increase thecontact between the base of the crown and the surface of the bone. Thisis because the load can only be distributed over the areas of the crownthat are in contact with the surface of the bone. An uneven contact withthe bone may cause the load to be increased over the areas of the crownthat are in contact with the bone. This is undesirable for many reasons,for example, because it does not allow the crown to distribute the loadin a predictable manner. This may have many detrimental effectsincluding, but not limited to, cracking, chipping, or breaking of thesurface of the bone.

In one embodiment, the crown may be designed such that it is capable ofbeing deformed by a predetermined amount. In this embodiment, a surgeonwould be able to visually inspect the surface of the bone and thendeform the crown by a desired amount in order to conform to the surfaceof the bone, as illustrated in FIG. 4D. In one embodiment, the crown maycomprise a material that is malleable using a given amount of force.This preferably occurs before the crown is placed in contact with thesurface of the bone. However, it is undesirable to allow the crown to bedeformed by, for example, the insertion forces of the screw. In oneembodiment, the force required to deform the crown is greater than theinsertion forces of the screw. This allows the crown to be deformed by asurgeon prior to placement in contact with the surface of the bone, butto maintain its shape subsequently. The desired deformation may beachieved using in any desired manner. For example, in one embodiment thedeformation may be achieved using a machine or instrument that iscapable of generating the required amount of force to deform the crown.

In another embodiment, the crown may be deformable based on one or moreincisions, or slits, that are formed in the base of the crown. FIG. 5Ais a diagram showing a bottom view of an exemplary base of the crown. Inthis embodiment, two or more diagonal slits 54-56 are formed in the baseof the crown. The slits 54-56 are preferably formed from the opening tothe outer perimeter of the base of the crown. The slits may allow thecrown to have greater flexibility and bending while still distributingloading over a larger area than the fastener or screw head. Someflexibility of the crown may permit it to better engage with an unevenbone surface, which may allow the base of the crown to achieve maximumcontact with the surface of the bone.

In the FIG. 5A embodiment, two diagonal slits 54-56 are formed. However,more than two slits may be used, as desired according to a particularapplication, and they need not all be oriented in a generally radialdirection as shown in FIG. 5A. For example, one or more notches on thebottom or top surface of the crown may allow the crown to be flexed orbent to conform to the shape of the bone, as shown in FIG. 5B. In otherembodiments, it may be desirable to have slits formed in otherdirections. For instance, in one embodiment, it may be desirable to haveone or more circumferential slits formed in the base or on the topsurface of the crown. The radial slit is preferably formed at apredetermined distance between the opening and the outer perimeter ofthe base of the crown, such as in a central region between about 25% toabout 75% of the distance from the center of the crown to its outerperipheral edge. One advantage of a circumferential slit is that itallows the crown to be deformed at selected areas around the perimeterof the crown, while allowing other portions to maintain their givenshape or contour. This may be particularly useful in applicationsinvolving irregularly shaped bones, such as those in the cervical spine.

In other embodiments, it may be desirable to have the slits formed nearthe interface between the fastener and the crown, as shown in FIG. 5D.In the FIG. 5D embodiment, two or more diagonal slits are formed fromthe opening to the outer perimeter of the top of the crown. As shown inthe FIG. 5D diagram, the slits allow the crown to be deformed accordingto the contour of the bone. In other embodiments, radial slits may beconfigured and dimensioned near the interface between the fastener andthe crown. In each of the FIG. 5A-5D embodiments, the slits allowsufficient deformation to match the contour of the surface of the bone.However, it is desired that the crown is sufficiently rigid to resistthe insertion forces of the fastener.

In one embodiment, the present invention is used to distribute the loadgenerated by inserting a fastener into bony tissue. As discussed above,the fastener may be a screw. Any type of screw may be used, depending ona particular application. FIGS. 6A and 6B are illustrations of oneembodiment of a fastener or screw that may be used with a crownaccording to the present invention. The fastener or screw preferably hasa spherical head 58 that is selectively engageable with a sphericalcurvature 46 of the crown, discussed in more detail below. An elongateshaft 60 is connected to the spherical head 58 to allow it to penetratebony tissue of the vertebrae. Preferably, the elongate shaft 60 includesthreads that aid in inserting the screw into bony tissue and holding itin place. In some embodiments, the screw may include a projection, forexample, a hexagonal projection, to aid in gripping the screw.

The top portion of the crown is preferably configured and adapted toreceive the screw. In some embodiments, it is desirable to insert thescrew into the bone at an angle, that is not perpendicular to thesurface of the bone. In such embodiments, it is still desirable for theload to be distributed evenly by the crown. In order to evenlydistribute the load, it is desirable for the base of the crown to remainin contact with the surface of the bone regardless of the angle ofinsertion of the screw. One way to do this is through the use of apartial ball and socket interaction. In other words, the surface of thecrown around the opening may be configured to have a sphericalcurvature. The spherical curvature preferably allows the interfacebetween the spherical head 58 of the screw and the crown to be flush fora variety of angles of insertion.

FIG. 7A is a diagram showing a side view of an exemplary crown. In thisembodiment, the opening comprises a surface having a sphericalcurvature. The spherical head of the screw preferably interacts with thespherical curvature of the crown, in order to provide flush contactbetween the two surfaces. In one embodiment, tangents to the sphericalcurvature of the opening diverge above the upper surface of the crown,as shown in FIG. 7B. This allows the spherical head 58 of the screw toremain flush with the spherical curvature of the opening even at largeangles of insertion.

In some embodiments, tangents to the curvature of opposing points alongthe spherical curvature of the crown may intersect at a location abovethe upper surface of the crown, as shown in FIG. 7C. One advantage ofhaving the tangents to the curvature intersect is that the sphericalcurvature of the opening generates an interference area. As the head 58of the screw is being screwed into place, a sufficient amount of forcemay be applied to force the head 58 of the screw to contract slightly.As the screw continues into the bone, the head 58 of the screw is ableto pass through the interference area. Once the head 58 of the screwpasses through the interference area it fits into the sphericalcurvature of the opening. It is desirable that forces that cause thescrew to backout are not strong enough to force the screw back throughthe interference area. The resistance of the interference area may bemodified by changing increasing or decreasing its curvature.

Returning to FIG. 7C, the tangents to the spherical curvature mayintersect to form an angle. This angle is preferably between about 0.1and about 20 degrees, or alternatively may be from about 0.5 to about 10degrees. In another embodiment, the angle between the tangents isbetween about 1 and about 5 degrees, while in yet another embodiment theangle may be between about 1 and about 3 degrees.

Another advantage of the spherical curvature shown in FIGS. 7A-C is thata screw may be inserted into bony tissue at any desired angle. In otherwords, the screw does not have to be inserted perpendicular to thecrown. The interference area, caused by the spherical properties of thehead 58 of the screw and the spherical curvature of the opening, arepreferably sufficient to prevent the screw from backing out. Whileopening prevents the screw from backing out, it allows it to rotatefreely. One advantage of allowing the screw to rotate freely is that thecrown according to the present invention is able to accommodatemovements of bone, for example, cervical vertebrae. Another advantage ofallowing the screws to be inserted at any angle is that it allowsrelatively close spacing of the screws, without the risk of interferencewith one another.

In order to allow the screw to be inserted at any desired angle, theopening is preferably large enough to allow for a wide range of motion.As described above with respect to FIGS. 4A-4E, the opening may comprisea variety of shapes. Regardless of the shape of the opening, the totalarea formed by the opening is preferably large enough to accommodate theshaft 60 of the screw. However, the opening is preferably small enoughthat it does not permit the spherical head 58 of the screw to completelypass through the opening. Because of the varying shapes and sizes of theopening, it is convenient to describe the relationship between thespherical head 58 of the screw and the opening in terms of the diameterof the smallest part of the opening. In other words, the diameter of thesmallest part of the opening should be sufficiently small to prevent thespherical head 58 from completely passing through the crown. This isdesirable in some applications in order to allow a surgeon to remove thescrew when desired. Preferably, the diameter of the smallest part of theopening is about 30% or less smaller than the spherical head 58 of thescrew. More preferably, the diameter of the smallest part of the openingis about 15% or less smaller than the head 58 of the screw, and mostpreferably the diameter of the smallest part of the opening is about 5%or less smaller than the head 58 of the screw. The relationship betweenthe spherical head 58 and the opening can also be expressed according tothe size of the head 58 relative to the opening. Preferably, thediameter of the spherical head is between 5% and 40% larger than thediameter of the smallest portion of the opening. More preferably, thediameter of the spherical head is between about 10% and about 30%larger, and most preferably the diameter of the spherical head isbetween about 15% and 25% larger than the diameter of the smallestportion of the opening.

In many applications, it is desirable to prevent backout of the screw.One way to do this is by creating an interference area, as describedabove. In another embodiment, a plurality of ridges may be selectivelypositioned along the inner surfaces of the opening. In some embodiments,a complimentary set of ridges are selectively positioned along thesurface of the spherical head 58 of the screw. One advantage of theridges is that they create interference between the crown and the screwhead such that the crown would also have to turn in order for the screwto back out, or to be unscrewed. This may be advantageous inapplications involving, for example, the cervical spine. In theseapplications, screws are often used to connect a plate or other deviceto one or more vertebrae. Backing out of a screw often occurs in theseapplications because of the movement of the vertebrae. Thus, preventingbacking out using ridges or an interference area is highly desirable.

It may also be desirable for the load distribution crown to graduallyincrease or decrease application of pressure on the bone as the fasteneris deployed further into or out of the bone. One benefit of this featureis that the physician may have greater control over the degree ofpressure applied to the bone from the fastener head. One way that thesefeatures may be provided in a load distribution crown is to configurethe crown with a spring-like mechanism that, when compressed or turned,applies progressively greater forces against the bone until reachingmaximum deflection. FIGS. 8A-D illustrate, several non-limiting examplesof how a load distribution mechanism may be configured with a springthat deflects and applies a force to the surface of the bone as thefastener is inserted.

FIGS. 8A and 8B, for example, show that a portion of the upper surfaceof the crown that communicates with the fastener head may have one ormore tabs or protrusions that can deflect under axial loading by thefastener. Thus, as the fastener is turned or otherwise moved furtherinto the bone, the tabs or protrusions on the upper surface of the crownwill bend or deflect under the loading until the fastener head is infull contact with the upper surface of the crown. The upper surface ofthe crown may have recesses formed therein corresponding to the shape ofthe one or more tabs or protrusions so that as the tabs or protrusionsare deflected they can move into the recesses. In one embodiment, thefully deflected tabs or protrusions are substantially flush with theremaining surface of the crown in contact with the fastener head.

Furthermore, an edge of the tabs or protrusions may be configured toprovide one or more tactile or audible signals to the physician toindicate the degree of deflection that has already occurred or thatremains before full deflection. For example, an edge of one or more tabsor protrusions may have a detent that at least initially interferes withan edge of the recess. As the fastener is driven further into the bone,however, the forces applied on the tabs or protrusions eventually maycause the detent to move beyond the interference area, therebyindicating to the physician that further progress of the fastener intothe bone may cause damage to the bone, cause the fastener to lose itspurchase in the bone, or both.

In another embodiment, illustrated in FIGS. 8C and 8D, the crown isconfigured to have a slit or cut extending from its outer perimeter edgeto the inner perimeter edge that defines the opening through which thefastener is inserted. The cut or opening may be further defined by twofree ends of the crown. When a slit or cut is present, one end of thecrown that defines the cut or slit may be disposed higher than theopposing end of the crown. As the fastener is inserted through theopening and into the bone, the head of the fastener may apply pressureto the surface of the crown and cause the crown to flex or bend, therebycausing the free ends of the crown to move toward a common plane. In oneembodiment, the free ends of the crown may move from about 0.1 mm toabout 5 mm before reaching a common plane. In another embodiment, thefree ends may move from about 0.5 mm to about 3 mm before reaching acommon plane. The flexing of the material that forms the crown allowsfor the storage of potential energy in the crown that can be released ifthe screw backs out. In addition, the use of a slit or cut allows forgradual increasing and decreasing of load distribution by the crown asthe fastener is inserted into or removed from the bone.

In one embodiment where a cut or slit is provided in the loaddistribution crown, such as illustrated in FIG. 8D, the free ends of thecrown may be configured to provide a tactile or audible signal to thephysician about the degree to which the crown has flexed or deformed.Thus, in one embodiment of the invention, the free ends of the crown areconfigured to provide three or more audible signals to the physician. Inone embodiment, at least one free end of the crown has at least onedetent or protrusion that interferes with a portion of the opposing freeend of the crown. As the free ends move toward a common plane orposition, the detent or protrusion is capable of providing tactile oraudible signal to the physician.

In one embodiment, the opposing free end of the crown has one or moredetents configured such that moving the detent from the first free endof the crown past one or more detents of the opposing free end causes atactile or audible signal. In yet another embodiment, the opposing endmay have one or more recesses or channels such that moving the detentfrom the first free end of the crown past one or more recesses orchannels of the opposing end causes a tactile or audible signal. Someembodiments of the invention may combine one or more detents andrecesses or channels, on the same free end to provide a variety ofsignals. In one embodiment, the potential energy stored in the crownfrom its deflection is sufficient to overcome the interference caused byinterferences between the free ends that provide signals to thephysician. Thus, in some embodiments the crown is capable of at leastpartially moving back toward its initial configuration if a fastener isbacked out or if pressure on the crown is relieved for any reason.

In yet another embodiment where the crown is configured to provide aplurality of tactile or audible signals, the pressure or force needed tocreate one signal is greater than the pressure or force needed to createanother signal. For example, in one embodiment, the force or pressureneeded to be applied by the fastener to the crown to move the free endsof the crown to create an initial signal is less than the force orpressure needed to be applied by the fastener to the crown to move thefree ends of the crown to create a second signal.

In yet another embodiment where the crown is configured to provide aplurality of tactile or audible signals, the intensity of one signal isgreater than the intensity of another signal. For example, if the signalprovided is an audible “click” the signals may progressively increase inintensity to notify the physician of the degree of compression of thecrown between the fastener head and the bone. Thus, in one embodiment, afinal signal during compression may have the greatest intensity so thata physician is notified that further deployment of the fastener may riskdamage to the bone or loss of purchase of the fastener in the bone.

Though the present invention has been described with respect to severalembodiments, those skilled in the art will recognize that the inventionis capable of alternate embodiments within the scope of the appendedclaims.

1. An apparatus for distributing a load in a spinal fixation system,comprising: a fastener; a crown body including a plurality ofprotrusions and an opening extending longitudinally therethrough and aradial slit extending from the opening to an outer perimeter edge;wherein the opening is configured and dimensioned to distribute the loadcaused by insertion of the fastener into bony tissue, and wherein firstand second free ends of the crown body are positioned adjacent theradial slit and the first free end is disposed higher than the secondfree end.
 2. The apparatus according to claim 1, wherein the number ofprotrusions is about 40 or less.
 3. The apparatus according to claim 1,wherein the length of the protrusions is between about 0.1 and about 4mm.
 4. The apparatus according to claim 1, wherein the averagepeak-to-peak distance between the one or more protrusions is betweenabout 0.4 mm to about 2.5 mm.
 5. The apparatus according to claim 1,wherein the one or more protrusions include a base, wherein the averagebase to base distance between neighboring protrusions is between about0.1 and 1.5 mm.
 6. The apparatus according to claim 1, wherein theprotrusions comprise an angle, wherein the angle from a base of theprotrusion to a peak of the protrusion is between about 50 degrees andabout 70 degrees.
 7. The apparatus according to claim 1, wherein theopening comprises a spherical curvature configured and dimensioned toevenly distribute the load caused by the insertion of the fastener intothe bony tissue at a non-orthogonal angle.
 8. The apparatus according toclaim 1, wherein tangents to the spherical curvature of the openingintersect.
 9. The apparatus according to claim 1, wherein the totalsurface area of the base of the crown is about 10 square mm or greater.10. The apparatus according to claim 1, wherein the crown is capable ofbeing deformed by a predetermined amount.
 11. The apparatus according toclaim 10, wherein a force necessary to deform the crown is greater thanthe force necessary to insert the fastener into the bony tissue.
 12. Theapparatus according to claim 11, wherein the deformation is based on anincision.
 13. The apparatus according to claim 1, wherein the fastenercomprises a spherical head having a diameter that is between about 5%and 30% larger than the diameter of the opening.
 14. The apparatusaccording to claim 1, wherein an inner portion of the opening and thesurface of the spherical head comprise a plurality of ridges.
 15. Anapparatus for distributing a load in bony tissue, comprising: a crowncomprising one or more curved protrusions and an opening having aspherical curvature; and a fastener including a head that interfereswith the spherical curvature of the opening; wherein the fastener isselectively engageable with, the opening to allow the crown todistribute the load caused by the insertion of the fastener into bonytissue, and wherein the one or more protrusions include a base, whereinat least one or more slits are formed in a base of the crown is capableof being deformed by a predetermined amount, wherein the curvedprotrusions are oriented on the bottom surface of the crown body topermit rotational sliding in one direction more readily than in anotherdirection, wherein one or more tabs that are deflectable are positionedon an upper portion of the crown body, the upper portion of the crownbody being in communication with the fastener head and the one or moretabs are configured to deflect under axial loading by the fastener, andwherein one or more recesses are formed on the upper portion of thecrown body corresponding to the shape of the one or more tabs so that asthe tabs are deflected when in communication with the fastener, the oneor more tabs move into the recesses.
 16. The apparatus of claim 15,wherein an incision comprises a circumferential groove positionedintermediate the opening and the outer perimeter.
 17. The apparatus ofclaim 16, wherein the incision comprises one or more radial incisionsextending radially between the opening and the outer perimeter.
 18. Theapparatus of claim 15, wherein the first and second free ends compriseone or more protrusions configured and dimensioned to interfere with aportion of the opposing free end to provide tactile or audible feedbackto a user.